Hi all, We would like to present an idea to improve the performance of data journaling in ext4. Data journaling is expensive because data is written twice: once to the journal and once to the actual file system. Passing data through the journal provides consistency guarantees that ordered journaling mode cannot provide (for example, data journaling prevents a data block from being partially written). The idea behind Selective Data Journaling is simple: create a new journaling mode by modifying ordered journaling mode to journal data blocks which are already part of a file. Data blocks which are newly allocated are not part of the journal, and are written out before the journal blocks in accordance with ordered mode's ordering guarantees. If there is a crash before transaction commit, the only side effect is un-allocated data blocks getting written with new data. Selective Data Journaling provides a lot of the benefits of data journaling, at significantly lower cost. For workloads which mostly deal with new data blocks (any applications which update files via atomic rename), Selective Data Journaling can increase performance significantly. We came up with Selective Data Journaling during my PhD at the University of Wisconsin Madison [1]. I haven't inspected the ext4 codebase deeply since then, so this optimization may already exist. There may also be problems with this approach that we have not considered -- we are open to discussion. It may also be the case that nobody uses data journaling so the extra complexity is not worth it. If this is something you would like to see implemented, my student Jesus Palos (cced) is interested in doing so. We would like to discuss how best to implement this if you are interested. [1] http://research.cs.wisc.edu/adsl/Publications/optfs-sosp13.pdf Thanks, Vijay Chidambaram, UT Austin http://www.cs.utexas.edu/~vijay/
